The invention more particularly relates to hydraulic systems for automatic braking at preselected braking pressures of swing drives found, for example, in excavators and cranes. Swing drives are usually comprised of hydrostatic drive having a hydraulic pump and motor, and associated gearing and controls that direct the horizontal rotation of booms found on excavator and cranes.
Swing drive arrangements have utilized the direct control of pressure of fluid being supplied to a motor to control output torque. With output torque control, the operator selects the magnitude and direction of the motor output torque.
Such arrangements typically include a mechanical or electrical control of the direction and magnitude of movement of the valve that controls the swing drive. A typical arrangement is shown in U.S. Pat. No. 3,696,836.
Among the objectives of the present invention are to provide a hydraulic system which is capable of being operated remotely by utilizing a pilot-operated control valve.
Typically, the torque control of swing drive provides free swing or coasting of the boom on cranes. That is, in the absence of a command signal in the hydraulic system, the boom or the boom and load will coast to a stop due to frictional forces without excessive oscillation of the boom cable or the load.
Excavators, are usually arranged with flow control to provide blocked center braking of the boom. That is, the boom or the boom and load will immediately decelerate to a stop in the absence of a command signal. In this case, return flow from the motor is relieved at the motor work port relief valve setting. The blocked center braking allows rapid alignment of the boom and load and also provides for maintaining the boom stationary with the excavator operating on an inclined surface.
It is also desirable, under certain conditions of operation, to brake the swing drive at a preselected reduced pressure; i.e. a pressure setting below the relief valve pressure setting.
It has been found that some operators, who have had their initial training and experience on a free swing braking arrangement on cranes, express a preference for the free swing feature when confronted with the operation of an excavator provided with a blocked center braking arrangement. Conversely, some operators, who have had their initial training on an excavator with the blocked arrangement, express a preference for the blocked center arrangement when confronted with the operation of a crane with a free swing braking arrangement.
In view of the foregoing, it is an object of this invention to provide a hydraulic circuit arrangement for automatic braking at preselected pressures of swing drives wherein an operator may selectively choose, by means of a simple adjustment, a free swing braking arrangement, a blocked center braking arrangement, or reduced pressure braking anywhere between the free swing and blocked center braking arrangements.
Accordingly, there is disclosed herein a selective swing drive automatic braking arrangement for both a torque control hydraulic system and a velocity control hydraulic system.
The hydraulic system comprises a reversible variable displacement pump, hydraulic servo motor means for controlling the direction and amount of displacement of said pump, a fluid motor connected in parallel to said pump and adapted to be driven by said pump, a pilot operated directional valve for controlling the position and direction of said motor system, and means for sensing the displacement of said pump and applying a force to said directional valve opposite the force of the pilot pressure for moving said valve.
More specifically, the torque control hydraulic system arrangement comprises a reversible variable displacement piston pump including a charging stage, a swashplate operable for varying pump displacement, a swashplate stroking cylinder acting against a swashplate biasing cylinder connected to the charging stage for varying the angle of the swashplate, and a swashplate angle feedback valve for sensing the angular movement of the swashplate; a swashplate control valve for controlling fluid flow to the stroking cylinder and including a spring centered differential area spool having first and second large area annuli and first and second opposed small area ends; a hydraulic motor connected in parallel across the pump and to both first and second small area ends of the control valve for supplying the driving torque to the swing drive; a control pressure relief valve connected between tank and the control valve, the charging stage, and the biasing cylinder; a control pressure reducing valve connected in series with the relief valve between tank and the feedback valve and in parallel with the relief valve and the biasing cylinder; and a manually operated hydraulic remote control valve having first and second pilot pressure lines connected to the first and second annuli of the control valve and to the feedback valve through first and second shuttle valves, respectively.